Urea is the most common nitrogen containing fertilizer, its consumption worldwide has been considerably increased, from ˜20 millions tons in the early seventies to ˜100 millions tons at the beginning of twenty first century. Nitrogen is the basic constituent for any living system (protein).
Due to intensive farming and reduction of emissions of sulphur in the air by industry and subsequent supply to the ground via rain, modern agriculture requires sulphur in addition to nitrogen.
Good agricultural practice usually require N and S in a ratio 10/1 to 5/1 in order to answer to the crop demand, for example 150 kg N/ha/year of N and 30 kg S/ha/year.
Lack of sulphur results both in lower quantity and lower quality of crops, and sulphur deficiency is often reflected in the content and type of proteins. Sulphur is indeed a major element entering into the chemistry of the bio cells, in molecules such as amino acids (cystine, methionine, etc). It is also a catalyst for the photosynthesis and, in some cases, may improve the fixation of atmospheric nitrogen.
Sulphate ions are readily available to the plant, whereas elemental sulphur first has to be oxidised by ground bacteria.
For all these reasons, urea ammonium sulphate (UAS), obtained through mixing of urea and ammonium sulphate appears to be a highly interesting compound fertilizer comprising at the same time nitrogen on one hand, and sulphur in sulphate form, readily available for the crop, on the other hand. Some percent of elemental sulphur may be added in addition in order to have some slow release effect of sulphur through the agricultural season.
AS can be either added to the urea solution prior to solidification (granulation, prilling, . . . ) as finely ground crystals, or can be directly synthesized in the urea solution or in the recycled flow as described in this patent.
Sulphur can be added as a molten salt in the urea solution or co-sprayed with UAS in the granulator.
Commercial grades of UAS should advantageously contain between 5% and 15% of sulphur and between 31 and 40% of nitrogen, which represents proportions from 2:3 to 4:1 of urea to ammonium sulphate. The N/S ratio can in such way be tailored to the crop demand or to the market request.
Description of Urea Synthesis Processes
All the commercial production of urea is from carbon dioxide and ammonia. The reaction proceeds in two steps, first with the formation of carbamate and second with the dehydration of carbamate into urea and water.
Urea is synthesized starting from CO2 and NH3 as raw materials. Thanks to the operating conditions (temperature and pressure), CO2 and NH3 combine easily into carbamate, simultaneously dehydrated to give urea and water.
The reaction is in two steps:2NH3+CO2NH2COONH4[(carbamate]CO(NH2)2[urea]+H2O
Both reactions take place in the liquid phase, whereas the raw materials ammonia and carbon dioxide are under gaseous phase.
The reactions are not total, but equilibrated. Thus ammonia is introduced in excess to the stochiometry in order to increase the conversion yield into urea.
The conversion yield per pass achieved in a typical urea production process (e.g. Stamicarbon as described for example in the “fertilizer manual”, chapter IX, edition printed in 1998 by IFDC) is around 80% of the urea, which could be potentially synthesised (i.e. the whole CO2 converted into urea and the excess ammonia remaining). The conversion of the raw materials is not completed in one pass, therefore the processes can be:                a once through process,        partial recycle process,        total recycle process.        
Once through and partial recycle processes mean that the unconverted raw materials are used in another production, e.g. ammonia to be used in a co-production of ammonium sulphate or ammonium nitrate. In such case, the carbon dioxide, which is nothing more than a low value by product of the ammonia synthesis, is usually vented to the atmosphere.
In once through process (e.g. Mitsui Toastu), raw materials are pumped to the urea reactor at about 200° C. and 200 bars. Urea excess in the reactor can be of 100-110% and NIC ratio is around 3.5. About 35% of the ammonia is converted to urea. Unreacted ammonia is separated and recycled at high pressure. The reactor effluent contains about, 80% urea solution after carbamate stripping. A large amount of ammonia must be used in some other processes (as described for example in the “fertilizer manual”, chapter IX, edition printed in 1998 by IDC).
Any co-production means less flexibility and most new plants are based on the total recycle process; all the unconverted raw materials are recycled to the urea reactor. The philosophy for these processes is as follows:                first, urea is synthesised in a high pressure (usually between 13-220 bars) high temperature (usually between 150-210° C.) reactor, allowing the conversion into carbamate and urea of the raw materials,        second, the flow from the reactor is then submitted to successive stages of pressure lowering and decomposition of the carbamate. Ammonia and carbon dioxide with some water are therefore stripped off the solution and the remains in the solution is mainly urea and water,        third, this urea solution may be either crystallized or concentrated further until adequate concentration for finishing process (prilling or granulation).        
Subsequently, the urea-unconverted reactants are recovered thanks to pressure lowering, flash and stripping as mentioned previously. The gaseous streams are then condensed into a carbamate solution and pumped back to the urea synthesis reactor.
For instance in Stamicarbon urea synthesis process (total recycle process) NH3 and CO2 are converted into urea via ammonium carbamate at a pressure of approximately 140 bar and a temperature of 180-185° C. The molar NH3/CO2 ratio in the reactor is around 3. This results in a CO2 conversion of 60% and a NH3 conversion of 41%.
The reactor effluent containing unconverted NH3 and CO2 is stripped at reactor pressure using CO2 as a stripping agent. The thermal effect and stripping effect lead to the decomposition of about 85% of the residual carbamate, and at a conversion of ˜80% of the carbon dioxide into urea in the liquid stream exiting the stripper.
The remaining NH3 and CO2 in the stripper effluent are vaporized in a 4 bar decomposition stage and subsequently condensed to form a carbamate solution, which is recycled to the synthesis section. Further concentration of urea solution takes place in the evaporation section where a e.g. 96% melt is produced to be sprayed in a granulator or prilled.
Unconverted ammonia depending on process conditions (pressure and temperature) may be found as carbamate, carbonates (different combinations of NH3 and CO2) or ammonia The signification of carbamate used hereafter has to be understood as unconverted raw materials in urea and does not correspond to the specific chemical compound but to a family of products depending on mole ratios, temperature and pressure, comprising ammonium carbonate, carbamate, sesquicarbonate, etc.
In some other processes (Snamprogetti, Mitsui Toatsu . . . ) higher N/C ratio, approximately 3.5 are operated in the reactor. The surplus ammonia is separated and recycled at high pressure. In these processes the condensation and recycling of ammonia excess as pure and water free is done to avoid water recycle. In these processes the condensation capacity of ammonia limit the urea reactor yield increase.
In Situ UAS Production
This unconverted ammonia can be neutralized by sulphuric acid to produce UAS.
AS synthesis reaction results from association of two ammonia molecules with one sulphuric acid molecule. Both AS and water are produced by the reaction. The heat of reaction, however, can be used as an energy source to evaporate water, for the concentration of the resulting urea ammonium sulphate (UAS) solution. AS synthesis reaction is particularly exothermic.
Two main technical steps have been identified and are required for UAS production starting from an aqueous urea solution: AS synthesis in urea and UAS solution concentration. On one hand AS synthesis is performed from reaction between sulphuric acid and ammonia (free or linked as carbamate). On the other hand the UAS solution is concentrated thanks to the heat of reaction released during the synthesis in the reactor and by an evaporator if necessary.
Successful and economical AS synthesis reaction and UAS production are closely related to operating conditions and process design. The invention considers new processes (3 routes) to produce UAS. The processes will be described hereafter. The invention concerns a complementary unit operation based on pipe reactor technology by which the synthesis reaction is performed.